An important trend in networking is the migration of packet-based technologies from local area networks (LANs) to metropolitan area networks (MANs). In the simplest terms, a MAN is a network that spans a metropolitan area. Generally, a MAN spans a larger geographic area than a LAN, but a smaller geographic area than a wide area network (WAN). The rapidly increasing volume of data traffic in MANs is challenging the capacity limits of existing transport infrastructures based on circuit-oriented technologies such as SONET, SDH, and ATM. Inefficiencies associated with carrying increasing quantities of data traffic over voice-optimized circuit-switched networks makes it difficult to provision new services and increases the cost of building additional capacity beyond the limits of most carriers' capital expense budgets. Packet based transport technology is considered by many to be one of the best alternatives for scaling metropolitan networks to meet the demand.
One leading packet based transport technology is Ethernet. Various different standard Ethernet interfaces operate at 10 Mbps, 100 Mbps, and 1 Gbps, and 10 Gbps, thus providing scalability of the service interface. Moreover, as nearly all Internet data packets begin and end as Ethernet frames, carrying data in a consistent packet format from start to finish throughout the entire transport path can eliminate the need for additional layers of protocol and synchronization that result in extra costs and complexities. In addition to efficient handling of IP packets, Ethernet has the advantages of familiarity, simplicity, and low cost. Although Ethernet is well suited for point-to-point and mesh network topologies, it can be difficult to deploy Ethernet in ring configurations and as a shared media. Ring network configurations act as a shared media and typically use media access control (MAC) mechanisms to manage access across multiple users. Ethernet, in contrast, has evolved to support full duplex switched infrastructures and lacks this type of MAC mechanism. However, much of the existing optical fiber network infrastructure in metro areas is in ring form, largely because incumbent transport technologies, e.g., SONET, are typically deployed over fiber rings. There are, therefore, great benefits in new technologies that can fully exploit fiber rings while retaining the inherent advantages of a packet-based transport mechanism like Ethernet.
A number of emerging technologies target metro data transport applications. Among these are the Dynamic Packet Transport/Spatial Reuse Protocol (DPT/SRP) and the IEEE 802.17 Resilient Packet Ring (RPR) standard. Dynamic Packet Transport is a resilient packet ring technology designed to deliver scalable Internet service, reliable IP-aware optical transport, and simplified network operations. DPT-based solutions allow service providers to cost effectively scale and distribute their Internet and IP services across a reliable optical packet ring infrastructure. DPT is based on SRP, which is a MAC-layer protocol developed by Cisco Systems for ring-based packet internetworking. The IEEE 802.17 RPR standard, offers several important features that have heretofore been exclusive to SONET: efficient support for ring topology and fast recovery from fiber cuts and link failures. RPR technology is expected to provide data efficiency, simplicity, and cost advantages that are typical to Ethernet. In addition, RPR technology solves problems such as fairness and congestion control that have not been addressed by incumbent technologies.
As outlined by the IEEE 802.17 RPR standard (the “standard”), the RPR layer model can be described in terms of the open systems interconnect (OSI) reference model familiar to those having ordinary skill in the art. A simplified block diagram showing the ring and station structure of an RPR implementation is shown in FIG. 1. A medium access control (MAC) control sublayer, a MAC datapath sublayer, and reconciliation sublayers are specified within the standard, as are the MAC service interface and PHY service interface supported by the sublayers. The MAC service interface provides service primitives used by MAC clients to transfer data with one or more peer clients on an RPR ring, or to transfer local control information between the MAC and MAC client. The MAC control sublayer controls the datapath sublayer, maintains the MAC state and coordination with the MAC control sublayer of other RPR MACs, and transfer of data between the MAC and its client. The MAC datapath layer provides data transfer functions for each ringlet. The PHY service interface is used by the MAC to transmit and receive frames on the physical media. Distinct reconciliation sublayers specify mapping between specific PHYs and the medium independent interface (MII).
Resilient packet ring system 100 includes a number of ring stations (station 0 130, station 1 140, station 2 150, . . . and station N 160) interconnected by a ring structure utilizing unidirectional, counter-rotating ringlets. Each ringlet is made up of links between stations with data flow in the same direction. The ringlets are identified as ringlet0 110 and ringlet1 120. This standard allows a data frame to be transmitted on either of the two connected ringlets. For example, a unicast frame is inserted by a source station and copied by the destination station. For efficiency, the destination also strips the now irrelevant stale frame. The portion of a ring bounded by adjacent stations is called a span, and thus a span is composed of unidirectional links transmitting in opposite directions. The RPR dual-ring topology ensures that an alternate path between source station and destination station(s) is available following the failure of a single span or station. Fault response methods include pass-through and protection, as described in the standard.
One common network element is a bridge. In general, bridges are devices with two or more network interfaces, that forward data frames from one interface to one or more of the other interfaces. The RPR standard specifies a MAC bridging reference model compliant with IEEE Std 802.1D-2004 (or IEEE Std 802.1Q-2003) transparent bridges, where the RPR network acts as a shared broadcast medium. Traffic may originate or terminate at either local or remote end stations, and may be forwarded across an RPR network to other 802 networks by transparent bridges. Local end stations are end stations that directly attach to an RPR network, while remote end stations are end stations which originate and terminate LAN traffic forwarded across an RPR network via transparent bridges. RPR stations operating as transparent bridges forward traffic between an RPR network and their other associated LAN networks.
FIG. 2 illustrates an architectural model of an IEEE 802.1D compliant RPR bridge. Each port of a MAC bridge connects to a single network. As shown, bridge 200 includes bridge relay entity 230 that interconnects the bridge's ports, at least two ports (210 and 220), and higher layer entities 240, such as bridge management, bridge protocol management (e.g., spanning tree protocol management), and the like. Bridge relay entity 230 handles the MAC method independent functions of relaying frames between bridge ports, filtering frames, and learning filtering information (235). A filtering database (FDB) or table 237 supports relay operations. Bridge relay entity 230 also uses internal sublayer services provided by the separate MAC entities for each port. Each bridge port transmits and receives frames to and from the network to which it is attached. In the example of bridge 200, two ports are illustrated, one for an RPR network and one for a generic network (e.g., another RPR ring or various other Ethernet networks). Each port supports appropriate MAC dependent functions 215 and 225.
In RPR networks, frames are normally stripped at their destination, i.e., to provide spatial reuse. This strategy cannot generally be used for remote frames, since the source/destination node is not on the ring. The RPR standard (see, Annex F) accommodates this situation by implementing so-called basic bridging. In basic bridging, all remote frames are broadcast (“flooded”) on the ring, so that they are seen by all bridges. In this way, RPR imitates a shared medium network, where all frames are visible to all nodes. However, this type of flooding prevents spatial reuse for remote traffic, and consequently consumes bandwidth resources. Efforts are currently underway as part of IEEE Project 802.17 to study proposals for an amendment to IEEE Standard 802.17-2004 that defines an addition to the 802.17 MAC to support spatially aware bridging. This amendment will be referred to as 802.17b, and will specify spatially aware or “enhanced” bridging.
While some bridges in RPR networks may include enhanced bridging functionality, it will still be desirable for those bridges to have the capability to interoperate with basic bridges. Accordingly, it is desirable to have mechanisms by which basic bridges can more efficiently be used with enhanced bridges. Moreover, it is desirable that such mechanisms operate, to the extent possible, within existing and emerging ring transmission schemes.